1. Field of the Invention
The present invention relates generally to an offset electrostatic imaging process, and more particularly, to such a process in which a dielectric imaging member prepared with a plasma spraying or detonation gun deposition technique can be advantageously employed.
2. Description of the Prior Art
In a typical electrostatic imaging process, a latent electrostatic image is formed on a dielectric charge retentive surface using a non-optical means, such as an electrostatic print head which generates ions by the corona discharge from a small diameter wire or point source. The dielectric surface can be either on the final image recording or receiving medium or on an intermediate transfer element, such as a cylindrical drum.
The latent electrostatic image is developed by depositing a developer material containing oppositely charged toner particles. The toner particles are attracted to the oppositely charged latent electrostatic image on the dielectric surface. If the dielectric surface is on the final recording medium, the developed image can then be fixed by applying heat and/or pressure. If the dielectric surface is on an intermediate transfer element, however, the developed image must first be transferred to the final recording medium, for example plain paper, and then fixed by the application of heat and/or pressure. Alternatively, the developed image may be fixed to the final recording medium by means of the high pressure applied between the dielectric-coated transfer element and a pressure roller, between which the final recording medium passes. Because not all of the developer material transfers to the final recording medium during the pressure transfer step, a residue of developer material will remain on the dielectric surface.
The intermediate transfer element in an offset electrostatic imaging process is typically a cylindrical drum made from a non-magnetic, electrically conductive material, such as aluminum or stainless steel, which is coated with a dielectric material. Suitable dielectric materials include polymers, such as polyesters, polyamides, and other insulating polymers, glass enamel, and aluminum oxide, particularly anodized aluminum oxide. Dielectric materials such as aluminum oxide are preferred to layers of polymers because they are much harder, and therefore, are not as readily abraded by the developer materials and the high pressure being applied. Anodized aluminum oxide layers have been particularly preferred as dielectric layers because they have smoother, less porous surfaces which are less likely to become clogged with developer material residue after repeated use.
Methods of depositing aluminum oxide layers on the conductive drum surfaces other than anodizing an aluminum drum, such as flame or plasma sprayed aluminum oxide, have been suggested. However, these other methods have never been adopted in practice because the aluminum oxide layers produced by flame or plasma spraying techniques are very porous and rough surfaces become very readily clogged with developer material residue. When the porous dielectric layer becomes clogged with developer material residue, the dielectric drum fails because the surface becomes laterally conductive, and thus, incapable of retaining an electrostatic latent charge image.
In order to maintain the dielectric properties of the porous dielectric layer, it has been found desirable to seal the pores with a polymer, such as epoxy, or a metal salt of a fatty acid, such as zinc stearate. The sealant prevents moisture from being absorbed by the porous layer which would cause the layer to become more conductive and less able to retain an electrostatic charge. The sealant improves the dielectric properties and also improves the release properties which permit the developed electrostatic image to be more completely transferred under pressure. Any moisture present in the porous dielectric layer should be removed prior to sealing using heat, vacuum, dessication, or a combination thereof.
Developer material residue can be cleaned from a sealed anodized aluminum oxide dielectric layer after each use by using a doctor blade to scraped the surface. Although anodized aluminum oxide dielectric layers have been found to be harder and have longer lifetimes than many other types of dielectric layers, they still are worn down or abraded by repeated use and become less capable of retaining an electrostatic charge.